The Unforgivable Sin
Many worry they committed the "unforgivable sin". I hear people ask this question all the time. I even heard one woman ask this question at a Bible study: "Were Adam and Eve kicked out of the garden because they had sex? Which was the unforgivable sin? Was the forbidden fruit, really 'having sex' symbolized by a tree-fruit?". Btw the woman was Catholic. While this is obviously an extreme case, there are many similar concerns out there for sins of the flesh.
To put your mind at ease, Jesus made it clear that the unforgivable sin is a spiritual one, that all sorts of sins of the flesh are forgivable.
Jesus said: 28 Truly I tell you, people can be forgiven all their sins and every slander they utter, 29 but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.”
30 He said this because they were saying, “He has an impure spirit.”
Notes: There you have it. Jesus stressed that the only sin that is unforgivable, is when a person is sure that another person is filled with the Holy Spirit, but calls them demon-possessed. Specifically in these verses, Jesus was talking about the Jewish "teachers of the law" that were there. He knew they recognized Him as the Messiah, but did not want to acknowledge Him. Instead they called Him demon-possessed. Which means they called the Holy-Spirit inside Jesus a "demon". Which is not a commonly committed sin today; usually people worry about sins of the flesh. So rest assured, it is unlikely that you did anything like this, and therefore your sins will be forgiven when you turn to God and accept Jesus as the Christ and savior. Then repent and start growing spiritually on the right path.
The same message of the unforgivable sin also showed in these two references:
Therefore, I tell you, people will be forgiven every sin and blasphemy, but the blasphemy against the Spirit will not be forgiven. Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the one to come.
1 John 5:16-17
“If anyone sees a fellow believer committing a sin that doesn’t lead to death, he should ask, and God will give life to him — to those who commit sin that doesn’t lead to death. There is sin that leads to death. I am not saying he should pray about that. All unrighteousness is sin, and there is sin that doesn’t lead to death.”
So how does one commit the unforgivable sin? Let's look at the above verses again. These were Jewish Pharisees who just witnessed Jesus perform a miracle of driving out a demon from a man. Then they called Jesus demon-possessed. In other words, they knew it was a Spirit from God but still called Him a demon. To apply this to today, it would have to be something like, someone has performed a miracle in the name of Jesus. And you called him demon-possessed. Not that likely to happen given the current zero number of people performing miracles in the name of Jesus. Of course this will present a dilemma in the future for those who are alive when the anti-Christ shows up on the scene, and starts performing wonders in front of people. This is why we should all keep seeking God, so we recognize the false prophets and not fall in that trap.
I see many internet articles from good Christian sources explaining the unforgivable sin as, "one lashes out at the Holy Spirit within one's self". This is not good explanation. The Bible verses were clear about describing this sin as people blaspheming against the Holy Spirit in others, not in their own self. Trying to connect those who become Atheists to the unforgivable sin is not the right explanation. It's two different things. The next section below addresses this.
Related Unforgiveness Clarifications
There is another way to become permanently unforgiven in today's world, and is happening often in young generations. If a person decides "there is no God" - he/she don't believe in God, and become an atheist for example. Then never turn back to God, and dies as an atheist? This person cannot be forgiven by God. This person basically cut the cord with God. God cannot force Himself on them. This person essentially sent himself/herself to hell. Remember, James 4:8 Come near to God and He will come near to you. And the opposite is true - move away from God and He will move away from you. And when God moves away from you, you have nothing left except utter darkness and torment.
So becoming an Atheist is forgivable if the person turns back to God, while alive.
But committing the "unforgivable sin" is never forgivable, no matter what the person does after.
The unforgivable sin (Blasphemy against the Spirit), on the other hand is never forgivable. But it is not the same as becoming an atheist. It is when calling another person, who’s obviously filled with the holly Spirit, demon possessed. This attitude puts such persons in a state in which they cannot repent. As they cannot repent, God cannot forgive them.
Not the Unforgivable Sin, but Equally Deadly:
There are several parables from Jesus on similar permanently unforgiven scenarios, for example, thinking I'm still young I have plenty of time to get right with God, as described in the parable below. Which teaches us even when complacent towards God at the time of our death, we are risking going to hell. No one knows when they're going to die. This is why we should always be right with God, grow spiritually, and be ready:
At that time the kingdom of heaven will be like ten virgins who took their lamps and went out to meet the bridegroom. Five of them were foolish, and five were wise. The foolish ones took their lamps but did not take along any extra oil. But the wise ones took oil in flasks along with their lamps. When the bridegroom was delayed, they all became drowsy and fell asleep. At midnight the cry rang out: ‘Here is the bridegroom! Come out to meet him!’ Then all the virgins woke up and trimmed their lamps. The foolish ones said to the wise, ‘Give us some of your oil; our lamps are going out. ‘No,’ said the wise ones, ‘or there may not be enough for both us and you. Instead, go to those who sell oil and buy some for yourselves.’ But while they were on their way to buy it, the bridegroom arrived. Those who were ready went in with him to the wedding banquet, and the door was shut. Later the other virgins arrived and said, ‘Lord, lord, open the door for us!’ But he replied, ‘Truly I tell you, I do not know you.’